1. Field of the Invention
This invention relates to new ceramic piezoelectric compositions, processes for preparing the piezoelectric ceramic materials and specific applications therefor.
2. Description of the Prior Art
A piezoelectric material is a material which generates a voltage when a mechanical stress is applied thereto. Moreover, when a voltage is applied to the piezoelectric material, mechanical deformation of the material will occur. Measures of piezoelectric activity include the "d" coefficients which relate electric charge and strain in the material. It is desirable to maximize these coefficients.
Perhaps the most promising group of piezoelectric materials are the family of complex niobates which have a perovskite or tungsten-bronze structure. This class of materials demonstrates outstanding electrical properties as single crystals. However, the use of single crystals for most ceramic applications is impractical. To date no one has been able to formulate acceptable polycrystalline materials of the ferroelectric perovskites which show the highest piezoelectric properties using standard ceramics technology. This is because non-piezoelectric phases, which are detrimental to the electrical properties of the ceramic, have been formed in addition to the desirable perovskite phase.
A polycrystalline ceramic material will display a piezoelectric effect if the material is anisotropic. In practice, polycrystalline piezoelectric materials are made anisotropic by heating the polycrystalline material to a temperature not far below the Curie temperature (Tc) of the material and thereafter cooling while in the presence of a strong electric field. This procedure will orient the otherwise randomly oriented dipoles of the polycrystalline material to result in a net distribution of positive and negative charges in the polycrystalline material (i.e., a dipole). Known piezoelectric materials include barium titanate, lead titanate, PZT (lead zirconate--lead titanate) and lead niobate.
The piezoelectric effect was discovered late in the 19th century and has been observed to occur in naturally occurring crystals such as quartz and Rochelle salt. However, the aforementioned polycrystalline ceramic materials have been focused upon by many researchers and have undergone constant improvement over recent years in an attempt to maximize electrical and physical properties thereof. The interest in polycrystalline ceramic piezoelectric materials is due to their use in such applications as transducers for sound (e.g., microphones, alarm systems), high power ultrasonic generators (e.g., sonar, ultrasonic cleaning), pick-ups and sensors (e.g., record players), resonators and filters (e.g., radio, television), etc.
Polycrystalline ceramic piezoelectric materials have received enormous attention because such materials can be readily formed into various shapes and sizes and thereafter poled (i.e., polarized) to result in a desirable net dipole orientation in the polycrystalline ceramic material. Moreover, ceramic piezoelectric materials borrow all of the desirable mechanical properties of ceramics generally, namely, high compressive strengths, good chemical resistivities, etc.
PZT is perhaps the most well known of the aforementioned ceramic materials (it is noted that PZT is a binary mixture of lead zirconate and lead titanate). PZT mixtures have been utilized for all of the various applications previously discussed, with minor compositional modifications being made to suit particular requirements. Attempts continue to be made to maximize the electrical properties of PZT ceramics, as well as the electrical properties of other ceramic compositions. Such electrical properties include dielectric constant (K), aging (i.e., piezoelectric deterioration with time), coupling coefficient, etc. However, as others have discovered, when one or more of the aforementioned properties is (are) maximized, one or more of the other properties may be adversely impacted. Accordingly, many researchers continue to search for an optimal polycrystalline ceramic piezoelectric material.
One of the many important applications for polycrystalline piezoelectric ceramics is as hydrophones in submarines for anti-submarine warfare. Particulary, piezoelectrics are used in targeting systems and in passive listening systems in submarines. When piezoelectric materials are used as hydrophones, piezoelectric properties such as g.sub.h (voltage coefficient), d.sub.33, d.sub.31 (piezoelectric strain coefficients in different crystallographic directions), d.sub.h (hydrostatic strain coefficient, wherein d.sub.h =d.sub.33 +2d.sub.31), and d.sub.h g.sub.h (figure of merit which relates to the sensitivity of the hydrophone) are all important. Moreover, it is desirable to maximize d.sub.h, d.sub.h g.sub.h and K values in a hydrophone. Particularly, while efforts have been made to maximize d.sub.h, known d.sub.h values for ceramics alone are not sufficient to be used in hydrophones for advanced sonar systems such as wide aperture and towed arrays. Furthermore, g.sub.h =d.sub.h /.sup..epsilon..sub.o K and K must be a large value For example, when K is a large value and the piezoelectric material is electrically connected to amplifiers, cables, etc., an inexpensive (i.e., non-complex) preamplifier can be used. However, if K is a relatively low value, then expensive (i.e., complex) preamplification equipment is required. Additionally, the figure of merit must also be sufficient. Moreover, g.sub.h must also be a relatively high value, but, for typical ceramic compositions, both g.sub.h and K can not be simultaneously maximized. Thus, to enhance both d.sub.h and g.sub.h, a composite material must be formed. It is clear that if d.sub.h g.sub.h is too low, the sensitivity of the hydrophone will be insufficient for its intended purpose. The figure of merit is particularly important because modern submarines are much quieter than their predecessors and employ accoustic/magnetic/electric signature reduction technologies. As a consequence, more sensitive acoustic systems are needed as alternative means for submarine detection.
There are also known various manufacturing processes for the formation of polycrystalline ceramic piezoelectric materials. Such processes include conventional mixing of oxides, molten salt synthesis of PZT materials, as disclosed by Arendt et al (U.S. Pat. No. 4,152,281), and Woodhead et al (U.S. Pat. No. 3,725,298) have disclosed the use of the so-called sol-gel process for forming PZT. However, many investigators continue to search for a desirable process for forming desirable polycrystalline piezoelectric ceramic materials.
Moreover, efforts have been made to synthesize single crystal materials such as PZT and PZN-PT (i.e., lead zinc niobate-lead titanate). However, while various piezoelectric parameters are known for PZN-PT single crystals, to date there has been little research directed toward PZN-PT polycrystalline ceramic materials.